A number of methods of removing NO.sub.x from NO.sub.x -containing exhaust gases such as combustion exhaust gases have been heretofore known. Of these, a catalytic reduction method using ammonia as a reducing agent is considered to be very useful since the method gives a satisfactory result even on treatment of exhaust gases having relatively high concentrations of NO.sub.x and oxygen. In the method, an NO.sub.x -containing exhaust gas is brought into contact with ammonia in the presence of a catalyst at a temperature at which the catalyst can exhibit its optimum activity. By the contact, the NO.sub.x is reacted with ammonia and coverted into harmless substances, i.e., N.sub.2 and H.sub.2 O. The reactions of NO.sub.x with ammonia will be expressed by the following formulae. EQU 6NO + 4NH.sub.3 .fwdarw. 5N.sub.2 + 6H.sub.2 O EQU 6no.sub.2 + 8nh.sub.3 .fwdarw. 7n.sub.2 + 12h.sub.2 o
a variety of catalysts are usable in the method and are different in temperatures at which they are optimumly activated. Most of the catalysts are activated in the temperature range of 200.degree. C.-400.degree. C. Some catalysts exhibit extremely high activity at relatively low temperatures of 100.degree. C.-230.degree. C. In an industrial sense, the reaction of NO.sub.x with ammonia should preferably be conducted at a velocity as high as possible and at a temperature as low as possible.